If you multiply the slopes of two perpendicular lines, what is the result?

2) if line a contains the points (3, -4) and (6, -2), and line b contains the points (-1, 5) and (1, 2), prove that the lines are perpendicular by multiplying their slopes.

3) line a and line b are perpendicular. line a contains the points (4,2) and (-1, 3). if one of the points on line b is (3, 7), find another point on line b.
(i got number 2 however dont know how to do 3) !